1. Technical Field
Example embodiments of the present invention relate in general to mobile communication, and more specifically, to an apparatus and method for automatically controlling a frequency in a mobile communication system.
2. Related Art
Recently, an orthogonal frequency division multiple access (OFDMA) scheme or a single carrier frequency division multiple access (SC-FDMA) scheme has been adopted for various mobile communication technologies. For example, IEEE 802.16, 802.20 and wireless broadband (WiBro) systems adopt the OFDMA scheme, and a long term evolution system (LTE) of 3rd generation partnership project (3GPP) adopts the OFDMA scheme for a downlink and the SC-FDMA scheme for an uplink.
In the case of using the OFDMA and SC-FDMA schemes, when a carrier frequency of a base station transmitting a carrier does not exactly match that of a terminal receiving the carrier, i.e., when there occurs a frequency error, noise occurs in every packet transmitted, causing degradation of performance. In order to remove the noise, according to a typical technology, a reference signal such as a pilot signal is transmitted to a particular resource space formed with a particular frequency and particular time so as to measure distortion of a channel, and this distortion is compensated.
According to a typical method for removing a frequency error, a carrier frequency difference between a base station transmitting a carrier and a terminal receiving the carrier is detected by measuring distortion of a channel, and a frequency of the carrier is compensated for as much as the carrier frequency difference. After the carrier frequency is updated, it is maintained until next update. Thus, this method may be used when a user moves at low speed, a data rate is low, or a power resource is not limited, i.e., when there is no limitation in retransmission of data.
However, when a user moves at high speed, and thus a carrier frequency difference is rapidly changed, the above-described method is limited in removing a frequency error as a Doppler shift effect cannot be considered.
According to another method for removing a frequency error, after a carrier frequency is once updated, a frequency error is predicted by using a previously updated value until next update, and the predicted frequency error is compensated for, thereby removing the frequency error even when the carrier frequency is rapidly changed.
However, according to this method, an overshoot or undershoot phenomenon occurs when a high rate of carrier frequency variation is changed into a low rate. Thus, this method is limited in correcting a frequency error.